This invention relates to an EGR (Exhaust Gas Recirculation) control system for controllably recirculating a portion of the exhaust gases passing through the exhaust passageway of an internal combustion engine back to the combustion chamber of the same, and more particularly to an improvement in the EGR control system of the engine equipped with an electronically controlled fuel injection system.
It is well known in the art that a part of the exhaust gases of an internal combustion engine is recirculated back to the combustion chamber of the engine in order to suppress the maximum temperature of the combustion taken place in the combustion chamber to reduce the emission level of nitrogen oxides (NOx) which are generated during the combustion in the combustion chamber. By virtue of this exhaust gas recirculation, the NOx emission level has thus effectively been lowered. However, the recirculated exhaust gas considerably affects the combustion in the combustion chamber and stability of the engine and therefore its amount is desired to be strictly controlled in response to engine operating conditions.
In this regard, the following EGR control system has been proposed: An EGR control valve is closeably disposed in an EGR passageway connecting an intake passageway and an exhaust passageway of an internal combustion engine. The EGR control valve is operated to control the amount of the exhaust gas recirculated back to the combustion chamber of the engine, which is accomplished by varying the exhaust gas pressure in the EGR passageway upstream of the EGR control valve in accordance with the variations of a venturi vacuum in the intake passageway. By this EGR control system, the recirculated gas flow is prevented from being affected by the variation of exhaust gas pressure in the EGR passageway. As a result, the recirculated exhaust gas flow can be controlled only in accordance with the venturi vacuum which is highly reliable as a function of the flow amount of the intake air conducted through the intake passageway into the combustion chamber. This EGR control system makes it possible to effectively decrease NOx emission level without causing the degradation of the driveability of motor vehicles.
It has now been carried out, with the above-mentioned EGR control system, to effectively lower the volume rate of the recirculated exhaust gas relative to the intake air inducted into the combustion chamber (this rate is referred to as "EGR rate") under a suburban area driving condition of the motor vehicle on which the EGR control system is mounted. Because, under such a condition, a relatively stable engine operation is maintained decreasing NOx generation in the combustion chamber, and less EGR seems to be suitable from view points of improving driveability and fuel economy of the vehicle during high speed cruising.
However, even by such an EGR control manner, difficulties have been encountered in which the combustion condition in the engine combustion chamber is abruptly changed to a great extent at a moment that the vehicle driving condition is changed from urban area driving condition into the suburban area driving condition and therefore the driveability of the vehicle is considerably degraded. Additionally, nearly no EGR is undesirable from a point of view of NOx emission control.